1. Field of the Invention
The present invention relates to an in-vehicle display device.
2. Description of Related Art
Japanese Patent Application Publication No. 2008-209724 (JP 2008-209724 A) describes an in-vehicle head-up display that reflects a display image on a display unit onto the front windshield to allow a driver to view the virtual image of the reflected display image. In the head-up display, a reflection screen that can be switched between the transparent state and the non-transparent state is provided on the front windshield and a display image (for example, an image of vehicle information such as the vehicle speed or the route guidance) is reflected on the reflection screen in the non-transparent state to show the driver the virtual image of the reflected display image. According to the head-up display, based on the fact that the driver's effective visual field is narrowed as the vehicle speed increases, the transparent state and the non-transparent state are switched for each part on the reflection screen according to the vehicle speed such that the virtual image is displayed in the upper position of the vehicle as the vehicle speed increases.
The spatial distribution of the real-space scene ahead of the vehicle is not constant in brightness and color tone and, in addition, the spatial distribution of brightness and color tone varies as the time elapses. Therefore, when an image is superimposed on the real-space scene ahead of the vehicle by means of the in-vehicle head-up display, the visibility varies because it is affected by the brightness and the color tone of the real-space scene ahead of the vehicle. The visibility of some of the images displayed on the in-vehicle head-up display is easily decreased by the effect of the brightness and the color tone of the real-space scene ahead of the vehicle. Therefore, there is a possibility that superimposing these types of images on the real-space scene ahead of the vehicle significantly decreases the visibility, makes it difficult to transmit information to the driver smoothly, and hinders the driver's visibility of the real-space scene ahead of the vehicle.